1. Field of the Invention
The present invention relates to a linear actuator wherein an electric motor drives a spindle and the spindle is provided with a spindle nut secured against rotation, which directly, as for example indicated in EP 0 787 267 B1 to Dietmar Koch (OKIN), or which over a tube-shaped actuation rod, as indicated in EP 1 322 876 B1 to LINAK A/S, is connected to an adjustable element for adjusting the same.
2. The Prior Art
The fully extended positions of the spindle nut can be determined in various ways. Typically this can be achieved by means of end-stop switches, which determine the inner and outer position of the spindle nut. As for instance mentioned in EP 1 322 876 B1, this can be brought about with a strip-shaped circuit board equipped with end-stop switches at each end. The circuit board is arranged in parallel to the spindle and in the track of the spindle nut, so that the spindle nut activates the switches in the respective outer positions. This requires a special construction of the guidance or protective tube (also known as outer tube) surrounding the spindle. The circuit board with the switches is connected to the control by wires or cables.
Another construction is mentioned in EP 0 647 799 B1 to LINAK A/S, where the end-stop switches on a circuit board are arranged in the housing of the actuator at the end of the outer tube, the outer tube being embedded in the housing against two springs in a longitudinally movable manner. In the outer positions the spindle nut engages a stop in the guidance tube, which activates the respective end-stop switches.
In the type of actuator first mentioned switches can be placed between the two end-stop switches for detecting intermediate positions. On the face of it, this is not possible by the last mentioned type.
A different way of stopping the spindle nut in the outer positions is by means of mechanical stops combined with a measurement of the electric current consumption. When the nut engages one of the stops, the consumption of electric current will rise considerably, and at a certain predetermined level cut off the current to the motor. The construction has the obvious disadvantage that it is heavy loaded in the outer positions. Here, it is not possible to detect an intermediate position either.
In order to continuously determine the position of the spindle nut, a rotary potentiometer or magnetic or optical encoders connected to the spindle or the transmission may be used. By counting the number of rotations combined with the thread pitch of the spindle, an indication of the position of the spindle nut is achieved. This can also be used for determining the outer positions of the spindle nut. This is of course a more expensive solution than simple end-stop switches. Moreover, the rotary potentiometer has the disadvantage that it can be turned excessively, which causes an incorrect determination of the position. The magnetic and optical encoder has the disadvantage that the position is lost if the spindle begins rotation on its own. When the actuator stops in an intermediate position, it can rotate a little backwards, or it can rotate backwards as a result of failure in the self-locking ability of the actuator, for example by over load or vibrations in the construction. Therefore the position determination is typically combined with end-stop switches.
The purpose of the invention is to provide a more production and logistic-friendly solution, where at the same time it is possible to detect intermediate positions.